1. Technical Field
The present invention relates generally to a steering control system for a vehicle. More particularly, the invention relates to a rear wheel steering control system for preventing tuck-in or drift-out from occurring due to sudden acceleration during turning.
2. Background Art
A Japanese Patent First Publication No. 55-91458 discloses a four-wheel steering (4WS) system for steering rear wheels in addition to front wheel steering control. This system is operable to steer rear wheels at the same phase as a steered angle of front wheels in a range of steered angle less than a preselected angle while it steers the rear wheels at a phase opposite the front wheels over that range. When a steering wheel is greatly turned for changing directions or making a U-turn for example, the rear wheels are controlled to be steered in the opposite phase to improve turning properties with respect to the possible minimum turning circle. Additionally, when the steering wheel is slightly turned for changing lanes for example during high speed travel, the rear wheels are steered at the same phase to generate cornering forces on the rear wheels positively improving steering stability.
However, such a vehicle has a drawback in that there is a tendency for the vehicle to become unstable if an acceleration pedal is operated suddenly during a steering operation of the front wheels. This means that in a case of a front-wheel drive (FWD) vehicle, releasing the acceleration pedal suddenly during turning causes tuck-in to occur which directs the vehicle to an inside direction where a turning radius becomes small or suddenly depressing the acceleration pedal causes drift-out which increases the turning radius.
A Japanese Patent Second Publication No. 64-6068 discloses a four-wheel steering system for avoiding such a drawback. This system is adapted for detecting engine load based on the amount of depression of an acceleration pedal to correct a rear wheel steering angle according to the detected engine load, thereby canceling tuck-in or drift-out. For detecting engine load, this conventional four-wheel steering system includes a throttle opening detection unit or throttle valve sensor disposed on an acceleration pedal or an engine throttle valve to correct rear wheel steering control based on the engine load. However, appropriate correction may be not obtained when a driver shifts a transmission during rear wheel steering.
Sudden operation of the acceleration pedal causes the engine load to vary, thereby causing engine torque to vary suddenly resulting in an undesired vehicle attitude change such as tuck-in or drift-out. The cause of the vehicle attitude change is not the engine torque, but the torque of the driven wheels. Driven wheel torque is defined by multiplying the engine torque by a torque ratio corresponding to a gear ratio of the transmission (for automatic transmissions, addition of a torque ratio of a torque converter is necessary). The driven wheel torque is variable in accordance with gearshift position or the gear ratio of the transmission.
Therefore, in prior art rear wheel steering systems, when the driven wheel torque is low at a high gearshift position regardless of great engine torque, over correction is made affecting the steering stability.